Combining ORDER BY and DISTINCT

Without a transformation, a statement that contains both DISTINCT and ORDER
BY would require two separate sorting steps-one to satisfy DISTINCT
and one to satisfy ORDER BY. (Currently, Derby uses sorting to evaluate
DISTINCT. There are, in theory, other ways to accomplish this.) In some situations, Derby can
transform the statement internally into one that contains only one of these
keywords. The requirements are:

The columns in the ORDER BY list must be a subset of the columns in the
SELECT list.

All the columns in the ORDER BY list are sorted in ascending order.

A unique index is not required.

For example:

SELECT DISTINCT miles, meal
FROM Flights
ORDER BY meal

is transformed into

SELECT DISTINCT miles, meal
FROM Flights

Note that these are not equivalent functions; this
is simply an internal Derby transformation.